The present invention relates to a machine for manufacturing a continuous web of paper or board, comprising a wet section, a press section and a drying section, said wet section including a forming fabric, and said press section comprising
a first press including a first press member, a second press member, said press members forming a first press nip with each other, a first press felt running in a loop through the first press nip, and a second press felt running in a loop through the first press nip, the first press member being situated in the loop of the first press felt and the second press member being situated in the loop of the second press felt, and
a second press including a first press member, a second press member, said press members forming a second press nip with each other, a first press felt running in a loop through the second press nip, and a second press clothing running in a loop through the second press nip, the first press member being situated in the loop of the first press felt and the second press member being situated in the loop of the second press clothing.
A paper machine of the type described above is known through U.S. Pat. No. 4,483,745. In the first press of the known paper machine, the lower press felt passes around the suction roll for a long distance that is more than twice as long as the actual suction zone. The pressed paper web is transferred to the upper press felt of the second press by means of a suction roll forming a transfer nip with the suction roll of the first press. Suction means cannot therefore be arranged before the transfer of the paper web to the second press to remove water from the lower press felt of the first press downstream of the first press nip. The paper web is therefore rewetted by the wet lower press felt when the paper web and the lower press felt run in intimate contact with each other around the suction roll said long stretch after the suction zone. Furthermore, the known construction lacks steam boxes before the press nip to increase the temperature in the press nip. Neither is it provided with blow boxes to improve running of the paper web.
U.S. Pat. No. 4,561,939 describes a paper machine having a press section consisting of two presses. However, both the first and the second presses lack suction rolls. Suction shoes are used to ensure that the paper web accompanies the desired press felt after the press nip, which makes the construction more expensive. A similar paper machine, having a press without suction press roll, is also described in U.S. Pat. No. 5,389,205.
EP-0 803 605 describes a paper machine with three presses, the middle one of which is located above the other two presses. The first press has a lower shoe press roll and an upper suction press roll around which the upper press felt runs some considerable distance to be deflected more than 90xc2x0 after the press nip. The suction press roll has a first suction zone situated opposite the press shoe, and a longer suction zone following the first suction zone and extending to the point where the upper press felt and the paper web leave the suction press roll in vertically upward direction. The upper press felt of the first press also forms the press felt in the first press nip of the middle press. A shoe press with a similar suction press roll and similar large deflection of the web-supporting press felt is described in DE-29701948-U1. In both cases the second press has only one clothing, namely a press felt, but lacks a second clothing, so that the paper web runs a longer distance around one press roll of the second press. A drawback with such configurations as those described in the latter two patent specifications is that the web runs in an open draw from the last press to the drying section and a considerable difference in speed is necessary there in order to remove the web from the smooth roll. This limits the speed of the machine to about 1700-1800 m/min.
Each of the known paper machines constitutes a specific configuration offering very limited opportunity or none at all, for choosing positions for the clothings and press members of the presses, and web run in accordance with the customer""s desires and specific machine spaces in each individual case.
The object of the present invention is to provide an improved paper or board machine designed so that rewetting of the web is avoided, which is free from open draws that limit the speed of the paper or board machine, which offers ample opportunity to apply various types of equipment such as suction boxes, blow boxes and steam boxes at suitable points along the web run, and which allows the positions of the press clothings and press members, as well as web run, to be chosen in accordance with the customer""s desires and specific machine spaces in each individual case.
The machine according to the invention is characterized by the combination
that the first press member of the first press is a suction roll with a suction zone, within which the first press nip is situated and which has an upstream end and a downstream end, said downstream end being situated downstream and spaced from the first press nip, the second press felt being arranged to be separated from the first press felt at a separation point situated upstream and spaced from the downstream end of the suction zone,
that the first press felt of the first press is situated either in a lower or an upper position, and the first press felt of the second press is situated in a reversed relationship in an upper position and lower position, respectively,
that the first press felt of the first press is arranged to carry the web from the first press nip to the second press,
that the first press member of the second press is a shoe press roll,
that the first press felt of the second press is arranged to carry the web from the first press felt of the first press to the second press nip, which is an extended press nip,
that a pickup suction roll is arranged in the loop of the first press felt of the second press, close to the first press felt of the first press for transfer of the web from the first press felt of the first press to the first press felt of the second press,
that the first press felt of the first press is arranged to leave the suction roll at a point situated at or close to the downstream end of said suction zone,
that suction means are arranged in the loop of the first press felt of the first press at at least one point downstream of the suction roll before the transfer of the web to the first press felt of the second press, and
that heat-supply means are arranged on the outside of the loops of said press felts situated in an upper position at points where one side of the web is exposed and the other side adheres to said press felts.